Problem: 10 rubber stamps cost $18.80. Which equation would help determine the cost of 14 rubber stamps?
Explanation: There are several equations that could help determine the cost, each with a slightly different approach. Let $x$ represent the unknown cost of 14 rubber stamps. Since 14 rubber stamps cost $x$ , we have the following proportion: $\dfrac{14}{x}$ We can write the fact that 10 rubber stamps cost $18.80 as a proportion: $\dfrac{10}{\$18.80}$ The cost changes along with the number of rubber stamps purchased, and so the two proportions are equivalent. $\dfrac{14}{x} = \dfrac{10}{\$18.80}$